The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with methods for the production of pulverulent coating compositions for high molecular weight post condensation and based on polyamides having at least ten aliphatically bound carbon atoms per carbonamide group or copolyamides or a mixture of homopolyamides and copolyamides containing at least 70% of the stated components while employing the precipitation procedure.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat. Nos. 3,476,711; 3,900,607; 3,927,141; 3,966,838; 4,143,025; 4,195,162; 4,273,919; and 4,334,056; British Pat. Nos. 535,138; 688,771; and 1,392,949; and the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105 particularly page 92 - polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Power Processing, pages 101-102, the disclosures of which are incorporated herein by reference.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822.
The use of polyamide based pulverulent coating compositions to prepare varnish type coatings for metals is known. The coating is implemented by the melt film method, that is, by fluidized bed, flame-spraying or electrostatic coating. The polyamide powders are obtained by precipitating the polyamide from solutions as disclosed in British Pat. No. 688,771, or by grinding the polyamide granulate, preferably at low temperatures in an inert gas atmosphere as in U.S. Pat. No. 4,273,919.
The precipitation procedure of British Pat. No. 688,771 comprises the precipitation of polyamide powders from ethanol by cooling the hot polyamide solution by active external means or by merely letting it stand. It is only by active cooling or cooling by standing of the hot polyamide solution that powders with a high portion of fine particles are obtained and these fine particles result in dust generation in the fluidized bed.
Pulverulent polyamides of high molecular weights are desirable as coating compositions in the light of their high toughness, especially where large metal parts are coated, in particular parts with sharp edges, generally speaking molded parts which are shaped to have very small radii of curvature.
Post condensation of the coating compositions takes place during coating. This requires that there be adequate amounts of an acid catalyst such as phosphoric acid in the polyamide powder.
Conventionally the coating powders of high molecular weights prepared by the known precipitation procedure contain from 0 to 0.2% by weight of phosphoric acid. However, this phosphoric acid concentration is too low to achieve an adequate rate of polycondensation for the post condensation during the coating procedure. Even when uncontrolled polyamides with equimolar amino and carboxyl end groups are used, the coatings cannot be sufficiently post condensed.
It is true that one may start with polyamides having higher molecular weights, that is, with those having a .eta..sub.rel (relative viscosity)=1.7, which contain adequate proportions of phosphoric acid, namely at least 0.4% by weight. However, when powders are prepared by the precipitation procedure and are based upon such polyamides, these polyamides are so decayed during dissolution and precipitation that powders made in this manner in spite of their post condensation are unsuitable as coating powders for thick walled metal parts because an inadequate coating on the edges results.
Accordingly, such powders to date have been conventionally prepared by the cold grinding method, that is, by the method of U.S. Pat. No. 4,273,919.
However, coating powders prepared by the grinding method incur a disadvantageous grain size distribution and their manufacture entails an additional and high cost grading procedure.
Lastly, such a method incurs undesired agglomerations which subsequently must be reduced by abrasion or grinding into the required powder size. Moreover, such a precipitation method results in non-reproducible, that is, differing batches which have a variation in particle size, molecular weight and bulk density.
It is further known to prepare polyamide powders by grinding polyamides of low molecular weights and then inducing in them the desired viscosity by heating them to temperatures below the melting point as disclosed in British Pat. No. 535,138 and U.S. Pat. No. 3,476,711.
Polylauryllactam powders also are prepared by this method and again are used in coating by procedures as disclosed in Chem. Ind. Nov. 1968, pp 783-791, and Modern Plastics, Feb. 1966, pp 153-156. Because polylauryllactam powders do not always meet the requirements of high elasticity, good edge coating, smooth surface, resistance to aqueous alkaline solutions, and often generate thick smoke in processing, the most diverse improvements have become known, such as polylauryllactam powders containing plasticizers as disclosed in U.S. Pat. No. 3,900,607, or those from a mixture of homopolylauryllactam copolyamides containing lauryllactam as disclosed in British Pat. No. 1,392,949, or those containing polyamides with N-alkoxymethyl groups besides acidically reacting catalysts as disclosed in U.S. Pat. No. 3,966,838, or mixtures of polyamides having 8 to 11 aliphatically bound carbon atoms per carbonamide group, aminoplasts bearing alkoxyalkyl groups and acidically reacting catalysts as disclosed in U.S. Pat. No. 3,927,141. In specific instances these powders exhibit good properties but they do not fully meet all requirements.
U.S. Pat. Nos. 4,143,025 and 4,195,162 describe an improved powder procedure. This procedure still fails to be fully satisfactory because the preparation of pigment free powders employs grinding and the preparation of pigmented powders entails precipitation. Lastly, success of this procedure presumes the use of polylauryllactam granulates prepared exclusively by hydrolytic polymerization in the presence of specified amounts of phosphoric acid.
U.S. Pat. No. 4,334,056 discloses a substantially improved procedure comprising a method of producing polyamide powder from polyamide having a relative viscosity of 1.4 to 1.8 as measured in 0.5% meta-cresol solution at 25.degree. C. and at least 70% of said polyamide consisting of polyamide with at least ten aliphatically bound carbon atoms per carbonamide group, comprising:
(a) dissolving said polyamide in at least twice the amount by weight of ethanol in a closed vessel at a temperature between about 130.degree. to 150.degree. C. to form a solution of said polyamide;
(b) cooling said solution to a precipitation temperature between about 100.degree. and 125.degree. C. and ceasing said cooling at said precipitation temperature;
(c) precipitating said polyamide powder from said cooled solution of (b) polytropically with agitation and under an inert gas atmosphere; and
(d) separating said precipitated polyamide powders of (c) from said ethanol.